diff --git a/AK/Tests/TestCircularDuplexStream.cpp b/AK/Tests/TestCircularDuplexStream.cpp
index 13bd4d405c..f784205db2 100644
--- a/AK/Tests/TestCircularDuplexStream.cpp
+++ b/AK/Tests/TestCircularDuplexStream.cpp
@@ -64,9 +64,8 @@ TEST_CASE(overwritting_is_well_defined)
 
     stream >> Bytes { bytes, sizeof(bytes) };
 
-    for (size_t idx = 0; idx < half_capacity; ++idx) {
+    for (size_t idx = 0; idx < half_capacity; ++idx)
         EXPECT_EQ(bytes[idx], idx % 256);
-    }
 
     for (size_t idx = 0; idx < half_capacity; ++idx)
         stream << static_cast<u8>(idx % 256);
@@ -84,4 +83,31 @@ TEST_CASE(overwritting_is_well_defined)
     EXPECT(stream.eof());
 }
 
+TEST_CASE(of_by_one)
+{
+    constexpr size_t half_capacity = 32;
+    constexpr size_t capacity = half_capacity * 2;
+
+    CircularDuplexStream<capacity> stream;
+
+    for (size_t idx = 0; idx < half_capacity; ++idx)
+        stream << static_cast<u8>(0);
+
+    for (size_t idx = 0; idx < half_capacity; ++idx)
+        stream << static_cast<u8>(1);
+
+    stream.discard_or_error(capacity);
+
+    for (size_t idx = 0; idx < capacity; ++idx) {
+        u8 byte;
+        stream.read({ &byte, sizeof(byte) }, capacity);
+        stream << byte;
+
+        if (idx < half_capacity)
+            EXPECT_EQ(byte, 0);
+        else
+            EXPECT_EQ(byte, 1);
+    }
+}
+
 TEST_MAIN(CircularDuplexStream)
diff --git a/Libraries/LibCompress/Deflate.cpp b/Libraries/LibCompress/Deflate.cpp
index b1f89b5f4c..25f69c737e 100644
--- a/Libraries/LibCompress/Deflate.cpp
+++ b/Libraries/LibCompress/Deflate.cpp
@@ -138,17 +138,18 @@ bool DeflateDecompressor::CompressedBlock::try_read_more()
         m_eof = true;
         return false;
     } else {
-        // FIXME: This assertion depends on user input.
-        ASSERT(m_distance_codes.has_value());
+        if (!m_distance_codes.has_value()) {
+            m_decompressor.set_fatal_error();
+            return false;
+        }
 
         const auto run_length = m_decompressor.decode_run_length(symbol);
         const auto distance = m_decompressor.decode_distance(m_distance_codes.value().read_symbol(m_decompressor.m_input_stream));
 
-        size_t nread = 0;
-        while (nread < run_length) {
-            const auto nreserved = min(run_length - nread, m_decompressor.m_output_stream.remaining_contigous_space());
-            auto bytes = m_decompressor.m_output_stream.reserve_contigous_space(nreserved);
-            nread += m_decompressor.m_output_stream.read(bytes, distance + nread + nreserved);
+        for (size_t idx = 0; idx < run_length; ++idx) {
+            u8 byte = 0;
+            m_decompressor.m_output_stream.read({ &byte, sizeof(byte) }, distance);
+            m_decompressor.m_output_stream << byte;
         }
 
         return true;